It is well known that most of the bodily states in mammals including most disease states, are affected by proteins. Such proteins, either acting directly or through their enzymatic functions, contribute in major proportion to many diseases in animals and man. Classical therapeutics has generally focused on interactions with such proteins in efforts to moderate their disease causing or disease potentiating functions. Recently, however, attempts have been made to moderate the actual production of such proteins by interactions with molecules that direct their synthesis, such as intracellular RNA. By interfering with the production of proteins, it has been hoped to affect therapeutic results with maximum effect and minimal side effects. It is the general object of such therapeutic approaches to interfere with or otherwise modulate gene expression leading to undesired protein formation.
One method for inhibiting specific gene expression is the use of oligonucleotides and oligonucleotide analogs as "antisense" agents. The oligonucleotides or oligonucleotide analogs complimentary to a specific, target, messenger RNA (mRNA) sequence are used. Antisense methodology is often directed to the complementary hybridization of relatively short oligonucleotides and oligonucleotide analogs to single-stranded mRNA or single-stranded DNA such that the normal, essential functions of these intracellular nucleic acids are disrupted. Hybridization is the sequence specific hydrogen bonding of oligonucleotides or oligonucleotide analogs to Watson-Crick base pairs of RNA or single-stranded DNA. Such base pairs are said to be complementary to one another. The oligonucleotides and oligonucleotide analogs are intended to inhibit the activity of the selected mRNA--to interfere with translation reactions by which proteins coded by the mRNA are produced--by any of a number of mechanisms. The inhibition of the formation of the specific proteins that are coded for by the mRNA sequences interfered with have been hoped to lead to therapeutic benefits. Cook, P. D. Anti-Cancer Drug Design 1991, 6,585; Cook, P. D. Medicinal Chemistry Strategies for Antisense Research, in Antisense Research & Applications, Crooke, et al., CRC Press, Inc.; Boca Raton, Fla., 1993; Uhlmann, et al., A. Chem. Rev. 1990, 90,543.
A serious deficiency of unmodified oligonucleotides for these purposes, is the enzymatic degradation of the oligonucleotides by a variety of intracellular and extracellular ubiquitous nucleolytic enzymes, hereinafter referred to as "nucleases."
Initially, only two mechanisms or terminating events have been thought to be operating in the antisense approach to therapeutics. These are the hybridization arrest mechanism and the cleavage of hybridized RNA by the cellular enzyme, ribonuclease H (RNase H). Cook, 1991, supra; Cook, 1993, supra; Uhlmann, supra; Walder, et al., Proc. Natl. Acad. Sci., USA, 1988, 85, 5011; Dagle, et al., Antisense Research & Development, 1991, 1, 11. It is likely, however, that additional "natural" events may be involved in the disruption of targeted RNA. Many of these naturally occurring events are discussed in Oligonucleotides: Antisense Inhibitors of Gene Expression, CRC Press, Inc., Boca Raton, Fla. (Cohen ed., 1989).
Hybridization arrest denotes the terminating event in which the oligonucleotide inhibitor binds to the target nucleic acid and thus prevents, by simple steric hindrance, the binding of essential proteins, most often ribosomes, to the nucleic acid. Methyl phosphonate oligonucleotides, Miller, et al., Anti-Cancer Drug Design, 1987, 2, 117-128, and .alpha.-anomer oligonucleotides are two extensively studied antisense agents that are thought to disrupt nucleic acid function by hybridization arrest.
The second "natural" type of terminating event is the activation of RNase H by the heteroduplex formed between the DNA type oligonucleotides or oligonucleotide analogs and the targeted RNA with subsequent cleavage of target RNA by the enzyme. The oligonucleotides or oligonucleotide analogs, which must be of the deoxyribose type, hybridize with the targeted RNA and this duplex activates the RNase H enzyme to cleave the RNA strand, thus destroying the normal function of the RNA. Phosphorothioate modified oligonucleotides are the most prominent example of antisense agents that are thought to operate by this type of antisense terminating event. Walder, supra and Stein, et al., Nucleic Acids Research, 1988, 16, 3209-3221 describe the role that RNase H plays in the antisense approach.
A number of chemical modifications have been introduced into antisense agents--oligonucleotides and oligonucleotide analogs--to increase their activity. Such modifications are designed to increase cell penetration of the antisense agents, to stabilize the antisense agents from nucleases and other enzymes that degrade or interfere with their structure or activity, to enhance the antisense agents' binding to targeted RNA, and to provide a mode of disruption (terminating event) once the antisense agents are sequence-specifically bound to targeted RNA, and to improve the antisense agents' pharmacokinetic and phamacodynamic properties.
To increase the potency via the "natural" termination events, the most often used oligonucleotide modification is modification at the sugar-phosphate backbone, particularly on the phosphorus atom. Phosphorothioates, methyl phosphonates, phosphoramidites, and phosphorotriesters have been reported to have various levels of resistance to nucleases. However, all reported modifications of the sugar-phosphate backbone, with the exception of phosphorothioates and phosphorodithioates, obliterate the RNase H terminating event. Cook, 1991, supra; Cook, 1993, supra; Uhlmann, supra. Heteroduplexes formed between RNA and oligodeoxynucleotides bearing 2'-sugar modifications, RNA mimics such as fluoro and alkoxys, do not support RNase H-mediated cleavage. These modified heteroduplexes assume an A form helical geometry as does RNA-RNA heteroduplexes which also do not support RNase H cleavage. Kawasaki, et al., J. Med. Chem., in press 1993; Lesnik, et al., Biochemistry, submitted 1993; Inoue, et al., Nucleic Acids Res., 1987, 15, 6131.
Other modifications to "wild type" oligo-nucleotides made to enhance resistance to nucleases, activate the RNase terminating event, or enhance the RNA-oligonucleotide duplex's hybridization properties include functionalizing the nucleoside's naturally occurring sugar. Sugar modifications are disclosed as set forth in PCT Application assigned to a common assignee hereof, entitled "Compositions and Methods for Detecting and Modulating RNA Activity and Gene Expression," PCT Patent Application Number PCT US91 00243, International Publication Number WO 91/10671, the disclosures of which are incorporated herein by reference to disclose more fully such modifications.
Other synthetic terminating events, as compared to hybridization arrest and RNase H cleavage, have been studied in an attempt to increase the potency of oligonucleotides and oligonucleotide analogs for use in antisense diagnostics and therapeutics. One area of research is based on the concept that antisense oligonucleotides with modified heterocyclic portions, rather than sugar-phosphate modifications, can be resistant to nucleolytic degradation, yet on hybridization to target RNA provide a heteroduplex that supports RNase H-mediated cleavage. Modifications in the heterocycle portion of oligonucleotides may not affect the heteroduplex helical geometry of sugar that is necessary for RNase H cleavage.
Accordingly, there remains a great need for antisense oligonucleotide compositions that are capable of improved specificity and effectiveness both in binding and modulating mRNA modulation or inactivating mRNA without imposing undesirable side effects. The present invention addresses these, as well as other, needs by presenting novel compounds, based on the purine ring system, that may be used as oligonucleotide intermediates. It has now been found that certain positions on the nucleosides of double stranded nucleic acids are exposed in the minor groove and may be substituted without affecting Watson-Crick base-pairing or duplex stability. Reactive or non-reactive functionalities placed in these positions can best initiate cleavage and destruction of targeted RNA or interfere with its activity.